1. Field of the Invention
The present invention relates generally to a light irradiating apparatus. More particularly, the present invention relates to a light irradiating apparatus having light emitting diode(s) used therefor as a light source for projecting a substantially parallel light onto a screen or the like. In addition, the present invention relates to a light source preferably employable for an optical radar system. Additionally, the present invention relates to an optical fiber preferably employable for the optical radar system.
2. Description of the Related Art
To facilitate understanding of the present invention, a conventional optical radar system as well as a conventional light irradiating apparatus having a light emitting diode used therefor as a light source will briefly be described below with reference to FIG. 15 to FIG. 17.
FIG. 15 is a schematic side sectional view of a conventional light emitting unit for use in an optical radar system which has been developed as an apparatus for measuring a distance from a motorcar running ahead of a rear motorcar. In the drawing, reference numeral 1A designates a light source, reference numeral 3A designates a light irradiating lens and reference numeral 4A designates an emitted light.
Usually, the light source 1A having a certain small surface is arranged at the position coincident with the focus of the light irradiating lens 3A. Thus, the light outputted from the light source 1A becomes an emitted light 4A which propagates while gradually radially expanding after it has passed through the light irradiating lens 3A. When the motorcar running ahead of the rear motorcar is located remote from the rear motorcar, the emitted light 4A sufficiently radially expands during its propagation, whereby a reflective plate mounted on the rear part of the motorcar running ahead of the rear motorcar is reliably illuminated with the emitted light 4A. With the conventional optical radar system constructed in the above-described manner, the motorcar running ahead of the rear motorcar can visually be recognized by a driver on the rear motorcar, and moreover, a distance between both the motorcars can be measured with excellent accuracy by measuring the time that elapses from the point of time, when the light is reflected at the reflective plate until the point of time when the reflected light returns to the rear motorcar, by using suitable measuring means mounted on the rear motorcar.
However, when the distance between both the motorcars is short, the emitted light 4A hardly expands in the radial direction. For this reason, there arises an occasion that the reflective plate on the motorcar running ahead of the rear motorcar is not illuminated with the emitted light 4A, depending on the positional relationship between both the motorcars. It is obvious that the motorcar running ahead of the rear motorcar fails to be visually recognized by the driver in the rear motorcar and the distance between both the motorcars can not be measured by any means.
FIG. 16 is a schematic side view of a conventional light emitting apparatus having a light emitting diode used therefor as a light source (hereinafter referred to simply as a LED light emitting apparatus) for projecting a substantially parallel light onto a screen or the like. In the drawing, reference numeral 1B designates a LED light source, reference numeral 3B designates a light irradiating lens, reference numeral 4B designates an emitted light and reference numeral 12B designates a screen.
With respect to the LED light source 1B for emitting a light from the whole surface thereof, a difference in optical intensity of the emitted light 4B arises depending on the positions where wire bonding portions and tip end portions are arranged. For this reason, even when the emitted light beam 4B outputted from the LED light source 1B is transformed into a substantially parallel light via the light irradiating lens 3B, the light 4B having a fluctuation in optical intensity thereof is projected onto the screen 12B or the like while it is adversely affected by a pattern of emission of the light 4B from the LED light source 1B. In addition, since the LED light source 1B has a large light emission angle, an optical system associated with the light irradiating lens 3B becomes unavoidably complicated in structure when the emitted light 4B is intended to be transformed into a substantially parallel light at a high optical efficiency.
FIG. 17 is a schematic side view of a conventional optical radar system for which a conventional optical fiber is used. The optical fiber 1C is basically constructed by a core portion 31C having a relatively large refractive index and a peripheral clad portion 32C having a relatively small refractive index, and the light L introduced into the optical fiber 1C via an inlet port 33C linearly propagates through the core portion 31C in parallel with an optical axis 14 of the optical fiber 1C or propagates therethrough while repeatedly being reflected along the full length of the boundary between the core portion 31C and the clad portion 32C to reach an outlet port 15C. Thereafter, the light L is irradiated from the outlet port 15C formed at a right angle relative to the optical axis 14 of the optical fiber 1C to the outside while expanding conically.
An angle with which the light L can be irradiated from the outlet port 15C of the optical fiber 1C is determined depending on the reflective index of the core portion 31C, the refractive index of the clad portion 32C or the like. Since the light L is conically irradiated at any rate, when the optical fiber 1C is used for an optical radar system, a motorcar 6 running ahead of a rear motorcar (not shown) is illuminated with a circular pattern so that a reflective plate 7 mounted on the rear part of the motorcar 6 is reliably illuminated with the conically expanded irradiated light.
While the motorcar 6 runs in the forward direction on the road surface, it is necessary that the light L is irradiated with a large angle as seen in the leftward/rightward direction but it is not necessary that the light L is irradiated with a large angle as seen in the upward/downward direction. In other words, a part of the light L excessively irradiated in the upward/downward direction becomes useless. This leads to a problem that a quantity of the light irradiated to the reflective plate 7 on the motorcar 6 to illuminate the same becomes low.
In view of the foregoing problem, there have been hitherto raised many requests for providing a LED light irradiating apparatus which assures that a light can be emitted from a LED light source with uniform optical intensity of the emitted light but without fluctuation of distribution of the optical intensity of the same attributable to a pattern of emission of the light from the LED light source.